The present invention relates to devices for continuously electrically monitoring the condition of a fluid in a vessel such as the condition of oil for lubricating and particularly the condition of crankcase or sump oil in an internal combustion engine or the lubricant in a power transmission device.
Shown and described in a copending application entitled xe2x80x9cFLUID CONDITION MONITORxe2x80x9d, Ser. No. 09/220,556 Filed Dec. 23, 1998 now U.S. Pat. No 6,278,281 in the names of Robert A. Bauer, Richard W. Hirthe, Mark H. Polczynski, Martin A. Seitz and James E. Hanson and assigned to the assignee of the present application is a device utilizing electro impedance spectroscopy techniques for monitoring in situ the condition of engine or transmission lubricant and providing an electrical indication of the condition. The device of the aforesaid patent application to Bauer, et al. describes measuring the electrical impedance of the fluid to be monitored at a first low frequency current and at a second high frequency current of at least one Hertz, computing the difference of the measured current as an analog of the impedances and comparing the computed difference with stored values for known fluid conditions to thereby determine the condition of the monitored fluid and to provide an electrical indication when the fluid condition reaches a predetermined threshold. The device of the aforesaid Bauer, et al. application describes a sensor probe having spaced parallel plates or interdigitated strips of a generally rectangular configuration disposed to extend from a plug or closure in the wall of the vessel containing the fluid. However, the disadvantage of the device described in the Bauer, et al. application is that the construction of the probe is difficult where a large plate area is required inasmuch as the spacing and area of the plates must be precisely controlled. The aforesaid Bauer, et al. device has the disadvantage of being bulky and somewhat cumbersome to install particularly in applications where the available access opening in the fluid vessel is small. Furthermore, the Bauer, et al. device has been found to be temperature dependant; and, it has been desired to achieve automatic temperature compensation or adjustment for the impedance measurements taken over the range of operating temperatures to which the fluid is exposed in service.
For motor vehicles in mass production it has long been desired to provide a way or means of continuously electrically monitoring in situ the condition of fluid in a vessel such as engine or transmission oil in the sump and to permit such monitoring to be accomplished in a way requiring no modification of the fluid vessel such as the engine block or crankcase or the transmission casing.
In addition, it has been desired to provide a way of predicting the Remaining Useful Life (RUL) of engine oil. Heretofore, it has been necessary to drain a sample of the engine oil and perform laboratory analyses, such as by High Pressure Differential Scanning Calorimetry (HPISC) to determine the amount of residual antioxidant components blended into the oil by the motor oil manufacturer. However, this is an expensive and time consuming procedure and can be performed only at selected intervals in the engine service life. Thus it has long been desired to provide a way or means of providing a vehicle on-board sensor capable of providing an electrical indication of the engine oil in real time.
The present invention provides a solution to the above-described problem of enabling continuous electrical monitoring in situ of the condition of fluid in a vessel such as engine crankcase or power transmission oil by electro-impedance spectroscopy techniques employing sensing of the changes in impedance by current measurement as an analog thereof between a pair of spaced parallel conductors at a first sub-Hertzian (low) and a second Hertzian (high) frequency. The impedance is computed from current measurements and the computed impedance adjusted for temperature variation and the difference in the impedance computed and the difference compared with stored values of impedance as a function of temperature for known fluid or lubricant conditions to determine the instant fluid conditions; and, an electrical indication is provided when a threshold condition has been reached. The sensor of the present invention has sensed changes of impedance difference of the fluid of over seven hundred percent (700%) from new to fully depleted fluid such as engine oil, which provides a high resolution sensitivity of measurement.
Alternatively, the probe can be excited by a fractional or low frequency alternating voltage and at a high frequency of at least one Hertz and the phase shift of the resulting current at the high and low frequency determined by a zero crossing detector. The reactive impedance or reactance may then be computed and the differential reactance compared with known values to determine the fluid condition.
The monitor of the present invention includes a sensor probe having the conductors comprising wires disposed or wound spirally, preferably helically, on an elongated mandrel near the distal end. The spirally wound configuration is sized and configured to be insertable through an existing dipstick hole in the engine or transmission. The proximal end of the mandrel extends outwardly of the existing dipstick aperture and has a relatively small casing or housing thereon which has disposed therein the electronic circuitry for impedance current measurement and the determination of the fluid condition from stored values of such measurements in a look-up table and providing an electrical indication signal upon the fluid reaching a critical threshold condition. The fluid condition monitor of the present invention is particularly suitable to automotive applications wherein the probe inserted into a dipstick aperture in the engine may be connected to the vehicle power supply for providing the electrical indication to a remote indicator provided on the instrument cluster for display to the vehicle operator.
The spirally wound electrodes of the probe of the present invention are preferably wound in a particular pitch for an appropriate number of turns and then the pitch is reduced to about half for the remainder of turns to provide the desired length of the electrodes for the impedance measurement. The electrodes include lead means extending to the proximal end for connection to the circuitry externally of the dipstick aperture in the vessel containing the fluid to be monitored.
The present invention thus provides a unique electrical monitor for insertion in an existing fluid dipstick aperture which permits retrofitting in field service by simply removing the manual dipstick and inserting a probe through the dipstick aperture and connecting the probe to a suitable power supply for energizing the circuitry provided on the probe.
An RTD temperature sensor is included on the distal end of the probe for providing a temperature measurement of the fluid, which measurement is provided to the circuitry for providing a signal input for adjusting the impedance computed for measured current for temperature based on stored values or a known relationship established for variations in impedance of the electrodes in the fluid with temperature. The difference in the impedance computed at the fractional or low frequency and the high frequencies is then computed and a comparison is made with the known values of impedance difference as a function of temperature for known fluid conditions. If desired, a pair of self-heated thermistors may be disposed in spaced relationship on the probe for detecting a fluid level below the desired threshold and providing a low-fluid defeat input for disabling the fluid condition impedance measurements and providing a low-fluid indication.